It is a known fact that because of the appreciable volatility and the solubility of mercury in hydrocarbons and water that many naturally occurring gaseous and liquid streams, as well as those derived from various processes, contain elemental mercury. For example, some natural gas streams and artificially produced gas, such as hydrogen produced by the electrolysis of brine, contain varying amounts of mercury. In recent years it has been determined that mercury in both the industrial and consumer environments causes an increased health risk to human and animal elements which are exposed to such mercury or to mercury-containing waste products which may be discharged into local rivers and/or lakes. In addition, mercury is known to cause corrosion problems in industrial equipment. Therefore, mercury which may be present in liquid and vapor streams must generally be removed so as to be at a safe level.
A number of processes are known for the removal of elemental mercury from gaseous streams. For example, U.S. Pat. No. 3,194,629 discloses a process for removing mercury from a gas contaminated with mercury comprising passing the gas over activated carbon impregnated with sulfur. As disclosed in the examples, the patentee prepared the carbon impregnated with sulfur by depositing elemental sulfur on activated carbon with a volatile solvent which was subsequently removed to provide the activated carbon impregnated with sulfur. Although the sulfur impregnated activated carbon did show adsorptive capacity for mercury, the product prepared therein had relatively low adsorption capacity for mercury and had a relatively low rate of adsorption of mercury.
The patentee in U.S. Pat. No. 3,194,629 discloses that the activated carbon appears to have unique intrinsic properties which particularly suit its use as a support for sulfur in the removal of mercury from the gaseous stream as compared against sulfur impregnated silica gel and alumina, which were " . . . much poorer absorbent(s) for mercury vapor than sulfur on activated carbon." Further, the adsorbent would probably exhibit other problems associated with activated carbon adsorbents, such as a high solubility of the impregnated sulfur in hydrocarbons (leaching) and adsorption of a high level of organic compounds.
U.S. Pat. No. 4,101,631 discloses a process for the removal of mercury vapor from gas streams wherein such gas streams contain water vapor and at least 0.02 micrograms of mercury vapor per normal cubic meter. The process comprises passing the gas stream through an adsorption zone containing a crystalline zeolitic molecular seive having a silica to alumina molar ratio of from 2 to 20 and having elemental sulfur loaded thereon in the amount of at least 0.5 weight percent. The process disclosed therein is superior to a process wherein sulfur is loaded on an activated carbon adsorbent. Although this process is substantially better than those theretofore disclosed, such process does not employ the use of a polysulfide-containing adsorbent composition for mercury and the advantages associated therewith. The stability and selectivity of such polysulfide-containing adsorption compositions will be discussed further hereinafter.
U.S. Pat. No. 3,849,537 discloses a method for purifying gases containing elementary mercury by use of a wash solution containing a mercury (II) ion concentration from about 0.02 grams per liter to the saturation point and a corresponding concentration of at least one anion selected from the group consisting of chlorine, bromine, iodine, and sulfate, wherein such anions form insoluble Hg (I) salts in the wash solution. Thus, the process involves the formation of Hg (I) salts by the disproportionation of Hg (II) and elementary mercury. Although such a process should provide for relatively good removal of mercury from such gaseous streams, the inconvenience associated with employing large amounts of wash solution and the necessity of starting with mercury in such wash solutions necessarily makes such a process both inconvenient and hazardous by use of such highly toxic wash solutions.
U.S. Pat. No. 4,094,777 discloses a process for removal of mercury in a gas or liquid wherein the process comprises the use of an adsorption mass consisting essentially of a solid dispersant or support selected from the group consisting of silica, alumina, silicates, aluminates and silico-aluminates which have been impregnated with copper. At least 30% of the copper is present in the sulfide form (CuS) in an amount of 2 to 65% of the weight of the mass. In addition, the adsorption mass contains from 0-5% silver in the sulfide state. Although such a copper sulfide impregnated adsorption mass may have certain desirable properties by virture of the affinity of copper sulfide for mercury, such an adsorption mass would be expected to have limited adsorption capacity owing to the limited amount of sulfur available in copper sulfide since only one half of the sulfur present in copper sulfide is available for reaction with mercury which results in a reduced oxidizing power toward mercury and adsorptive capacity.
The use of aluminosilicates containing transition metals in the ion-exchanged form for mercury adsorption is disclosed in Germany Offen. No. 2,841,565, wherein zeolites, such as Zeolites A, X and Y, are ion-exchanged with transition metals, such as silver, copper, zinc, cadium and nickel, prior to being contacted with a mercury-containing gas. It would appear that the zeolite is functioning primarily as a chemical adsorbent for the elemental mercury by providing for oxidation/reduction reactions involving mercury.
U.S. Pat. No. 3,516,947 discloses a method for preparing a hydrocarbon cracking catalyst for use in hydrocarbon reactions wherein the catalyst is disclosed to be a siliceous crystalline material impregnated with sulfur-containing free radicals. The catalyst is prepared by heating a siliceous crystalline material with sulfur, alkali metal polysulfides, alkaline earth metal polysulfides, a mixture of sulfur and an alkali metal sulfide or a mixture of sulfur and an alkaline earth metal sulfide at a temperature in the range of about 200.degree. C. to about 1200.degree. C. The product as disclosed and claimed by the patentee contains sulfur-containing free radicals trapped within the pores of the siliceous material and does not contain polysulfide compounds.